A suction pile is a thin walled steel cylinder, closed at least one longitudinal end, which is placed on the sea bed with a lower end and which penetrates the sub sea soil with the aid of suction (under-pressure) created within the cylinder. The creation of the suction can be with the aid of a suction source, such as a pump, that is mounted on, close to or at a distance (e.g. above the water surface, e.g. at a vessel) from the suction pile. The applied level of the suction can be e.g. at least substantially constant, smoothly increase or decrease or comprise a number of pulses. After use, the suction pile can be removed by creating an overpressure within the cylinder, e.g. by pumping in (sea) water.
Suction piles and their way of installing are known from e.g. U.S. Pat. No. 4,318,641 in the name of Shell. The above patent describes a method and apparatus for securing a tubular element to the bottom of the sea. It describes the steps of lowering the tubular element from the water surface to the water bottom, removably securing the pumping unit to the tubular element either before or after the lowering of the tubular element, closing the tubular element at the upper end, reducing the pressure within the tubular element by operating the pumping unit, so as to cause the tubular element to penetrate into the water bottom to a desired depth. Subsequently, the operation of the pumping unit is stopped and the pumping unit is disconnected from the tubular element and raised to the water surface to recover it.
U.S. Pat. No. 6,719,496 in the name of Shell describes a method for deploying a suction pile anchor in which the flood valves are opened on the top of the suction pile and the suction pile anchor is off loaded from the anchor boat and lowered it to the sea floor. A remote operated vehicle (ROV) with pump capacity is used to close the flood valves and to control the suction into the soil.
The known suction piles and installation methods employ valves on the top of the suction pile in order to accommodate the lowering of the suction pile during landing. The water pressure inside the pile rapidly increases while the pile is landing on the seabed when it is dropped (launched) from several meters above the seabed to penetrate into the soil. The open valves or vents on the top of the pile allow the trapped water to escape and therefore avoid building up a water pressure inside the pile so that large pressure differentials between the inside and outside of the pile are avoided. When the water cannot escape fast enough there will be a pressure build up that will cause a bearing capacity failure in the soil when the pile lands.
After the pile is landed, the vents need to be completely sealed in order to control the internal pressure. If the vents don't completely seal, it is not possible to depressurize the pile to let it penetrate the soil, or to over pressurise it to come back out of the soil. It is needed that each vent ensures a 100% sealing over the lifetime of an installed pile. Therefore the pile cover at the top end should be fitted with specific installation aids. This makes the operation expensive as known pile covers normally have 2 or more valves or costly closing devices which requiring ROV assistance operations, particularly in deep waters.
Normally a submerged pumping skid is attached to the vents and activated by a ROV. The pumping skid is removed after the pile is depressurized. This operation also needs expensive anchor handling tug boats which are provided with a heave compensation system to safely and smoothly land the piles on the seabed.